1. Field of the Invention
The present invention relates generally to sewage treatment methods, and more particularly to a method for dewatering primary and secondary sewage sludge prior to incineration or other disposal.
Raw sewage wastes may be treated by a variety of methods, generally classified as primary, secondary, and tertiary, prior to disposal. Primary treatment usually consists of mechanical or gravitational separation of the solid and liquid components of the sewage, resulting in a solid fraction (primary sludge) with a low solids content (typically on the order of 2% to 5% solids, which may be thickened up to about 10% solids by conventional thickeners) and minimal biologic break-down. Secondary treatment methods usually consist of biological break-down of the organic matter in the sewage to both oxidize the organic matter and inactivate pathogenic microorganisms. The resulting solid fraction (secondary sludge) also has a low solids content (typically 1% to 5% solids which may be thickened to typically 5% to 10% solids by conventional thickeners). Tertiary treatment generally refers to a supplementary treatment of the liquid effluent from a secondary treatment plant to further oxidize the organic materials or detoxify the water, prior to discharge into environmentally sensitive bodies of water. Tertiary treatment methods generally do not produce significant quantities of sludge.
The present invention relates generally to disposal of the sludge produced by the primary and secondary treatment of raw sewage. Millions of tons of such sludge are produced each year by waste water treatment plants in the United States alone. A number of disposal schemes are presently employed for such sludge, including ocean discharge, land fill, use as a fertilizer, and incineration. The present invention is particularly concerned with the incineration of both primary and secondary sewage sludges. Heretofore, incineration has been an effective disposal method, but generally suffers from relatively high capital and operating costs. To reduce operating costs, the sewage sludge is normally dewatered to lessen the amount of fuel required for incineration. The dewatered sludge, however, generally has a very pasty consistency, requiring the use of specially designed furnaces, such as fluidized bed furnaces and multiple hearth furnaces for incineration. It would therefore be desirable to provide improved methods for dewatering the sewage sludge which provides for both the highly effective removal of water (to reduce operating costs) and provide for a sludge which has an improved consistency which allows for incineration in conventional furnaces (to reduce capital costs).
2. Description of the Background Art
U.S. Pat. Nos. 4,397,100 and 4,339,882, each disclose methods for dewatering organic substrates, such as sewage sludge, by contacting the substrate with a first solvent, such as a short chain alcohol, followed by contacting with a second solvent, such as a long chain alcohol. The first solvent is miscible with the water, while the second solvent is miscible with the first solvent, but not the water. In this way, water may be extracted from the organic substrate and the first solvent thereafter recovered by the second solvent. A system must be provided for separating the solvents so that they can be recycled. U.S. Pat. No. 4,014,104, discloses a method for drying coal comprising introducing a solvent, such as methanol which displaces the water and is thereafter more readily dried from the coal. The solvent is recovered by an unspecified solvent recovery system. The following U.S. patents disclose sewage dewatering systems which do not employ solvent displacement: U.S. Pat. Nos. 4,717,484; 4,659,422; 4,500,428; 4,402,834; 4,377,486; 2,349,390; and 1,543,939.